A wideband code division multiple access (W-CDMA) base transceiver station (BTS) may transmit a radio frequency (RF) signal, which may be reflected, and/or attenuated by various obstacles and surrounding objects while propagating though a transmission medium. Each of these reflections may comprise an individual distinct path, or path. As a result, a mobile receiver may receive a plurality of individual distinct versions of the transmitted signal, at a plurality of distinct time instants. Each of the received plurality of individual distinct versions of the transmitted signal may be associated with an individual distinct path and be referred to as an individual distinct path signal. A time instant associated with an individual distinct path signal, and a time instant associated with a subsequent individual distinct path signal may comprise a time offset. Various of the plurality of individual distinct path signals may be received at a received signal power level that may vary among the received plurality of individual distinct path signals. A time offset may vary among the received plurality of individual distinct path signals. For example, a time offset associated with an nth individual distinct path signal, and an (n+1)th individual distinct path signal, may differ from a time offset associated with an mth individual distinct path signal, and a (m+1)th individual distinct path signal, for m≠n. A measure of a time offset may be referred to as temporal proximity.
The plurality of individual distinct path signals that may be received at the mobile receiver may be referred to as multipath signals, or a multipath. A medium through which a transmitted RF signal may be propagated is referred to as the RF channel, or a channel. Transmission impairments, or impairments, may be present in the channel that may introduce distortion, interference, and/or distortion as the transmitted RF signal propagates through the channel. The presence of transmission impairments may increase the difficulty of recovering a transmitted RF signal at a mobile receiver based on corresponding received multipath signals. The channel may be characterized by its RF bandwidth and whether the channel comprises a single path, or a multipath (frequency selective fading channel). A channel that comprises a single path may be referred to as a “frequency flat fading channel.” A channel that comprises a multipath may be referred to as a “frequency selective fading channel.”
During reception of a multipath, and subsequent recovery of the corresponding transmitted RF signal, a multiplicative complex factor, comprising gain and RF phase, may be applied to an individual distinct path signal. The numerical values associated with the gain and RF phase may vary as the mobile receiver is moved from one location to another. The multipath signals may be characterized according to their energy levels. Each signal-path has associate therewith a respective geometric distance, which causes respective copies of the transmitted RF signal to arrive spread over time. This time duration may comprise a plurality of time instants during which a plurality of individual distinct path signals that constitute a multipath may be received at a mobile receiver. This time duration may also be referred to as a “delay spread.” During reception of a multipath at a mobile receiver, a profile for the measured signal path energy may increase and decrease during a relatively short time period. This time period may be referred to as a path life time. Consequently, the transmission medium, through which a transmitted RF signal propagates, may comprise time varying characteristics that influence the multipath that is received at the mobile receiver. The multipath may comprise a plurality of individual distinct path signals that may be received at a mobile receiver based on statistical properties.
A rake receiver may be utilized to recover an estimate of a transmitted RF signal carried in received multipath signals. The rake receiver may implement a process that comprises generating an estimate for each received individual distinct path signal in the multipath. This generated estimate may be referred to as a channel estimate. A channel estimate that is generated based on an individual distinct path signal may be referred to as a path estimate. The process may further comprise descrambling the received multipath through application of a Gold code (GC) and despreading the resultant signal through application of an Orthogonal Variable Spreading Factor (OVSF) code. The GC and the OVSF code may be applied to the received multipath at a time instant that is time synchronized to a reception, by the mobile receiver, of an individual distinct path signal. The descrambling and despreading operation may be referred to as correlating. Circuitry that performs the correlating may be referred to as a correlator.
The rake receiver may comprise a plurality of correlators, wherein an individual correlator may be time synchronized to a reception, by the mobile receiver, of an individual distinct path signal. The correlator may be referred to as performing time tracking on an individual distinct path signal. Such operation may also be referred to as signal (waveform, or signature) matching. Following correlation a path estimate may be generated. The path estimate may be utilized to recover an estimate for at least a portion of the corresponding transmitted RF signal. Subsequently, the rake receiver may perform an operation on the received plurality of path estimates that comprises a time compensation, and combining.
Within the rake receiver, circuitry, referred to as a “finger,” may be assigned to process a received individual distinct path signal. The finger may perform the steps described above, during reception of the assigned received individual distinct path signal. A finger among a plurality of fingers in a rake receiver may be assigned to a corresponding one of a received plurality of individual distinct path signals that form component signals in a multipath. The output of the fingers may then be combined and further demodulated and decoded. The fingers may be implemented to receive and process as much of the received signal energy as practicable.
A considerable part of receiver design may involve managing the rake receiver fingers. A functional block known as a “searcher” may be adapted to locate new multipath signals and to allocate rake receiver fingers to the new multipath. The searcher may detect a path based on the amount of energy contained in a signal, identify that path if it carries user data, and subsequently monitor the detected path. Once the detected signal energy in a path is above a given threshold, a finger in the rake receiver may be assigned to the path and the signal energy level constantly monitored.
However, partitioning the received signal into several fingers, each of which may process and exploit energy in a single path, may have limitations. For example, a multipath may rarely be characterized by a distinct discrete time of arrival in connection with each received individual distinct path signal. As a result, the rake receiver may be inefficient at exploiting the power in received signals. In addition, utilizing this method may incur high processing overhead in managing the fingers. The total amount of time required to identify a path, assign a finger, and exploit the signal energy may account for 20-30% of the life span of a path. Once a finger is assigned to a path, detected energy on the path may be continuously monitored. However by the time the finger has been assigned, the path energy may be diminished, while energy may arrive at a different time. This may result in the rake receiver constantly searching for new paths and performing finger de-allocation/allocation cycles. A finger that is allocated to a path with diminishing power may represent misused resources in the mobile terminal, which may in turn lower performance of the mobile terminal.
Another limitation in the conventional rake receiver is known as finger merge or ‘fat’ finger. This is a phenomenon in which paths are in close temporal proximity of each other. If a time difference between received individual distinct path signals is below some threshold value, more than one finger, among a plurality of fingers in a rake receiver, may be assigned to a common individual distinct path signal. Also, as a result of the close proximity, a bias may be introduced into the processing of the received multipath, whereby the two or more fingers track a single received individual distinct path signal. This fat finger phenomenon may result in negative implications for system performance.
The assignment of more than one finger to a received individual distinct path signal may be a waste of system resources, as the additional finger or fingers may be better deployed receiving additional energy from another received individual distinct path signal in the multipath, or receiving energy from a signal transmitted from another BTS. In addition, the combined power of the various fingers may often be used to control various system parameters, for example, power control. Without accounting for finger merge, a system may overestimate the received power due the duplication of energy detected in the combiner, and thus overcompensate by lowering transmit power to a threshold level below that required for adequate communication. Moreover, combining the output of merged fingers with the output of non-merged fingers may weight both the signal and noise of the merged finger output too heavily in relation to the non-merged finger output, which may result in inefficient exploitation of the received power.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.